I have recently bought a rear derailleur identical to the one that I had previously. after fitting it, I noticed that instead of changing into gear 1 as normal, it pushes into the gear one cog as it has not stretched down far enough and so i can not get into that gear. Is there something wrong or is there a simple fix that I should know about? cheers

don’t quite understand what you mean but it sounds like adjustment.

Check the park tools site or sheldon brown for how to set up a mech

http://www.parktool.com/blog/repair-help

http://www.sheldonbrown.com/

Sounds like you need to get the cable tension right. There are tons of tutorial videos on youtube. videos

basically the derailleur moves sideways into (and hits) the largest rear cog on the cassette, making it impossibe to change into gear 1.

Take the mech off, there’s a stopper on the mech bolt that twists around, this should rest on the mech hanger to stop it twisting around too far. Simples 🙂

What derailleur is it ?

b tension I think its called – again check the links above